Dr. Heidegger first shows his friends the old rose that has been revived to full bloom to give them a visual of his proposed experiment in order to pique their interest.
Understandably, the friends are not convinced. Dr. Heidegger then suggests that he, himself, will not be participating in the experiment, showing them that he has nothing to gain from their acceptance.
As the listen more closely, the doctor considers his audience by giving them a warning to be very careful in deciding. He is not using the strong-arm approach but making them be responsible for their own decisions. He warns them that returning to their youth will bring with it a responsibility to change all of their sins to virtues.
This particular strategy shows that he knows his friends well and knows they would respond to this type of challenge. As they agree, he compliments them, saying he knew they were the very best of carefully selected choices, even though they were his only choices.
By knowing his friends, Dr. Heidegger was certain he could get them to participate in his experiment.